Don't overdo the texture in the dirt... it covers big areas, so you don't want to get that repeating texture effect, or distract the player from the important areas of the screen.
What I'd do is add a second color, just a little darker than the colour you have now, and spatter it in there, like you would a grass tile, and clean it up so that you don't get noticeable tiling... then I'd do a couple of special tiles with little bits of rock or other things buried in the dirt... then place some of those at random amid the dirt tiles to break up the monotony.
The grass could use a highlight colour, at least. The round rock needs some dithering, because it currently looks too flat on the front surface, whereas I suspect you intended it to be more like a spherical boulder.